linux-kernel - Re: [PATCH v5 03/22] cc_platform: Add new attribute to prevent ACPI memory hotplug

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Date:   Wed, 29 Jun 2022 21:13:21 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Igor Mammedov <imammedo@...hat.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kvm-devel <kvm@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Len Brown <len.brown@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        isaku.yamahata@...el.com, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v5 03/22] cc_platform: Add new attribute to prevent ACPI
 memory hotplug

On Wed, 2022-06-29 at 10:48 +0200, Igor Mammedov wrote:
> > Hi Rafael,  Igor,
> > 
> > On my test machine, the acpi_memory_device_add() is not called for system
> > memory.  It probably because my machine doesn't have memory device in ACPI.
> > 
> > I don't know whether we can have any memory device in ACPI if such memory is
> > present during boot?  Any comments here?
> 
> I don't see anything in ACPI spec that forbids memory device being present at
> boot.
> Such memory may also be present in E820, but in QEMU is not done as linux used
> to
> online all E820 memory as normal which breaks hotplug. And I don't know if it
> still true.
> 
> Also NVDIMMs also use memory device, so they may be affected by this patch as
> well.

AFAICT NVDIMM uses different device ID so won't be impacted.  But right there's
no specification around "whether firmware will create ACPI memory device for
boot-time present memory", so I guess we need to treat it is possible.  So I
agree having the check at the beginning of acpi_memory_device_add() looks
incorrect.  

Also as Christoph commented I'll give up introducing new CC attribute.

> 
> > 
> > And CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED is only true on TDX bare-metal
> > system,
> > but cannot be true in Qemu guest.  But yes if this flag ever becomes true in
> 
> that's temporary, once TDX support lands in KVM/QEMU, this patch will silently
> break usecase.

I don't think so.  KVM/Qemu won't expose TDX to guest, so this code won't be
true in guest.

-- 
Thanks,
-Kai